1. Field of the Invention
This invention relates to a catalyst capable of selectively oxidizing carbon monoxide present in hydrogen-containing gases fed to the anode of a solid polymer electrolyte fuel cell, a method for eliminating carbon monoxide by using such a catalyst, and a solid polymer electrolyte fuel cell system using the catalyst.
2. Description of the Prior Art
Solid polymer electrolyte fuel cells have a high output density, are drivable at a low temperature and may emit little exhaust gases which contain injurious materials, and hence attract notice as a transport means energy source that can substitute for conventional internal combustion engines.
In fuel cells, hydrogen gas or a fuel gas containing hydrogen is fed to a fuel electrode (the anode) and air or oxygen-containing gas to an oxidizer electrode (the cathode), where hydrogen is oxidized as shown by the following equations, to generate electricity.
Anodic reaction:
H2xe2x86x922H++2exe2x88x92
Cathodic reaction (in the case of hydrogen):
1/2O2+2H++2exe2x86x92H2O
Overall reaction (in the case of hydrogen):
H2+1/2O2xe2x86x92H2O
On the anode and cathode, electrode catalysts are used in order to accelerate the respective electrode reactions. Electrode catalysts conventionally used include those comprised of platinum alone or combination of platinum with at least one selected from palladium, rhodium, iridium, ruthenium, osmium and gold, or combination of platinum with at least one selected from base metals such as tungsten, chromium, manganese, iron, cobalt, nickel and copper, which are used in the form of metal powders or alloy powders. Also, those comprised of any of these metal powders or alloy powders supported on conductive carbon particles have been used.
In fuel cells, hydrogen-enriched gases are commonly used which are obtained by previously reforming a fuel, e.g., an alcohol or hydrocarbon by means of a reformer. However, on electrodes of solid polymer electrolyte fuel cells drivable (operable) at a temperature of 120xc2x0 C. or below, carbon monoxide present in such hydrogen-enriched gases may poison the platinum contained in the anode electrode catalyst to cause polarization greatly, resulting in a decrease in output. In order to prevent this, it has been proposed to use the platinum contained in the anode electrode catalyst, in the form of its alloy with rhodium, iridium, ruthenium or the like (D. W. Mckee and A. J. Scarpellio Jr., J. Electrochem. Tech., 6(1969), p.101). However, even this method has its limitations for improving anti-CO-poisoning performance of the anode electrode catalyst. Anode polarization caused by being poisoned with carbon monoxide may greatly occur when the carbon monoxide in the hydrogen-enriched gas is in a concentration higher than 100 ppm.
Reformed gases obtained by subjecting oxygen-containing hydrocarbons such as methanol or hydrocarbons such as gasoline and methane to steam reforming, autothermal reforming or partial-oxidation reforming in the presence of water and/or air also contain carbon monoxide in a proportion of a few % to tens of %. This carbon monoxide is converted into hydrogen and carbon dioxide by allowing it to react with water by means of a water-gas shift reactor installed in a reformer or at the latter stage of the reformer. Such water-gas shift reaction, however, is an equilibrium reaction, and a reverse shift reaction which forms carbon monoxide and water from hydrogen and carbon dioxide may take place depending on reaction temperature. Hence, gases at the outlet (outlet gases) of the water-gas shift reactor usually contain, in addition to the chief components hydrogen and carbon dioxide, thousands of ppm to 1% of carbon monoxide and, in some cases, nitrogen. In order to keep the anode electrode catalyst of the solid polymer electrolyte fuel cell from being poisoned with carbon monoxide, concentration of this carbon monoxide must be lowered to 100 ppm or less, and preferably 50 ppm or less.
U.S. Pat. No. 5,248,566 discloses a method of selective oxidation in which, in order to lower the concentration of carbon monoxide, air which contains oxygen in an amount substantially equimolar to the carbon monoxide present in the outlet gases of the water-gas shift reactor is added to that outlet gases and the gas thus formed is brought into contact with a catalyst of rhodium/ruthenium supported on alumina to selectively oxidize the carbon monoxide without oxidizing the hydrogen in gases. As catalysts for selective oxidation of carbon monoxide present in hydrogen gas, catalysts comprised of a noble metal such as platinum, rhodium or ruthenium supported on a metal oxide such as alumina and silica are known as those for purifying hydrogen used for the synthesis of ammonia (Japanese Post-examination Publication (Kokoku) No. 39-21742). Recently, besides the catalyst disclosed in the above U.S. Pat. No. 5,248,566, also disclosed in the field of fuel cells for automobiles are a catalyst of ruthenium supported on titania (Japanese Laid-open Publication (Kokai) No. 8-295503) and a catalyst of ruthenium supported on zirconia (Japanese Laid-open Publication (Kokai) No. 10-101302).
However, such conventional catalysts for selective oxidation of carbon monoxide have insufficient activity and selectivity, and have had to be more improved in performance. In the case of fuel cells for automobiles, in order to mount a carbon monoxide selective-oxidation reactor on a car in its limited space, a catalyst is necessary which is as small as possible and yet exhibits a high carbon monoxide elimination rate. In the above conventional catalysts, an attempt to achieve a high carbon monoxide elimination rate has resulted in too large a size for the catalyst, and an attempt to make the catalyst small an insufficient carbon monoxide elimination rate. Moreover, in the conventional catalysts, their carbon monoxide oxidation activity is inhibited by the water contained in reformed gases or by oxygen-containing hydrocarbons and hydrocarbons remaining in a trace quantity. Thus, even if they can exhibit a high activity to any simulated gases containing only hydrogen carbon monoxide and oxygen, they have absolutely insufficient activity to actual reformed gases.
The present invention was made in order to solve the above problems the prior art have. Accordingly, an object of the present invention is to provide a catalyst for selective oxidation of carbon monoxide present in hydrogen-containing gases, and a method for eliminating carbon monoxide and a solid polymer electrolyte fuel cell system which make use of such a catalyst.
To achieve the above object, firstly the present invention provides a catalyst for selective oxidation of carbon monoxide present in hydrogen-containing gases; the catalyst comprising ruthenium supported on an alumina hydrate.
Secondly the present invention also provides a method for eliminating carbon monoxide present in hydrogen-containing gases, comprising the steps of:
adding to a gas containing at least hydrogen and carbon monoxide and being richer in the hydrogen than the carbon monoxide on the basis of volume, oxygen in an amount necessary for oxidizing at least part of carbon monoxide present in that gas; and
subsequently bringing the gas to which the oxygen has been added, into contact with the above catalyst for selective oxidation of carbon monoxide.
Thirdly the present invention still also provides a solid polymer electrolyte fuel cell system comprising a fuel storage container, a reformer, a shift reactor, a carbon monoxide selective-oxidation reactor using the above catalyst of the present invention, and a solid polymer electrolyte fuel cell which are disposed in this order.
The catalyst of the present invention which comprises ruthenium supported on an alumina hydrate has a superior selective-oxidation activity to carbon monoxide present in hydrogen-containing gases. Hence, the carbon monoxide selective-oxidation reactor using the catalyst of the present invention can be made light-weight and small. Also, because of the use of the catalyst of the present invention, gases having a low carbon monoxide concentration can be fed to the latter stage fuel cell, and hence the anode electrode catalyst of the fuel cell can be kept from being poisoned with carbon monoxide and the anode electrode catalyst can enjoy a long lifetime.
The solid polymer electrolyte fuel cell system using the catalyst of the present invention is useful as a fuel cell system for automobiles.